The embodiments described herein relate to an ultrasonic imaging apparatus for obtaining an image of a subject to which a contrast agent is injected.
In recent years, an image of a contrast agent injected to a subject is obtained by using an ultrasonic imaging apparatus. Ultrasound examinations using a contrast agent includes an examination of destroying the contrast agent and observing ultrasonic waves generated at the time of destruction and an examination of, without destroying the contrast agent, repeatedly irradiating the contrast agent with ultrasonic waves and observing a harmonic content in reflected ultrasonic waves from the contrast agent.
The contrast agent is destroyed by being irradiated with an ultrasonic wave of high sound pressure. Therefore, at the time of repetitively irradiating the contrast agent with ultrasonic waves and observing reflected ultrasound, the sound pressure of the ultrasonic wave to be applied has to be suppressed to a destruction sound pressure or less.
The ultrasonic sound pressure in the subject is expressed by using an MI (mechanical index) as an index of an action on a living body by ultrasonic cavitation. The MI value shows an amount proportional to the negative sound pressure in the ultrasonic wave in the subject and reflects the sound pressure in the subject (refer to, for example, “Revised medical ultrasonic equipment handbook” edited by Electronic Industries Association of Japan, Corona Publishing Co., Ltd., January 20, 1997, pp. 52 to 58). An estimated maximum MI value in a subject calculated by using scan parameter values or the like at the time of performing imaging is displayed on a display screen. The operator adjusts the magnitude of a maximum drive voltage at the time of driving piezoelectric transducers so that the estimated maximum MI value does not exceed a value at which the contrast agent is destroyed.
However, in the background art, each time the scan parameter value used for imaging is changed, the magnitude of the maximum drive voltage has to be re-adjusted. Specifically, in the case of changing the scan parameter values such as the electronic focus position and the aperture width, the maximum sound pressure generated in the subject changes, so that the estimated maximum MI value of a display screen to be calculated also changes.
Therefore, in the case of changing the scan parameter values, the operator has to simultaneously re-adjust the maximum drive voltage so as to maintain the estimated maximum MI value constant. It is a factor of deteriorating the operability in an examination using a contrast agent.
Consequently, it is important to realize an ultrasonic imaging apparatus in which the estimated maximum MI value in a subject is maintained at a predetermined value also in the case of changing the scan parameter values.
It is desirable that the problem described previously is solved.